Chronic Photoinhibition Research Articles

Physiological changes promoted by a strobilurin fungicide in the rice-Bipolaris oryzae interaction

Strobilurins are among the most important fungicides that are used for plant disease control worldwide. In addition to their fungicide effect, strobilurins can also improve crop physiology. Nonetheless, the impact of azoxystrobin (Az), the main marketed strobilurin, on rice physiology is still unknown. Detailed gas exchange measurements and chlorophyll a fluorescence analysis were used to examine the Az effects on the photosynthetic performance of rice plants (cultivar Metica-1) either challenged or not with Bipolaris oryzae, the causal agent of brown spot. Az impaired carbon (C) fixation in the non-inoculated plants in a manner that was not related to photochemical or biochemical limitations, but rather to decreased stomatal conductance that limited the CO2 influx into the mesophyll cells. The photosynthesis of rice plants that were not sprayed with Az dramatically decreased upon B. oryzae infection, which was chiefly governed by photochemical and biochemical limitations. The energy surplus that was caused by limited C fixation in the rice plants that were treated with Az and inoculated with B. oryzae was thermally and effectively dissipated until 72h after inoculation. In Az absence, however, this mechanism was not sufficient to prevent chronic photoinhibition to photosynthesis. The inoculated plants were not able to fully capture and exploit the collected light energy, but these constraints were greatly limited in the presence of Az. In conclusion, Az impaired the photosynthetic performance of non-infected plants by diffusive constraints, but prevented, to a greater extent, the damage to the photosynthetic apparatus during the infection process of B. oryzae.

Sub-cellular location of FtsH proteases in the cyanobacterium Synechocystis sp. PCC 6803 suggests localised PSII repair zones in the thylakoid membranes.

SummaryIn cyanobacteria and chloroplasts, exposure to HL damages the photosynthetic apparatus, especially the D1 subunit of Photosystem II. To avoid chronic photoinhibition, a PSII repair cycle operates to replace damaged PSII subunits with newly synthesised versions. To determine the sub‐cellular location of this process, we examined the localisation of FtsH metalloproteases, some of which are directly involved in degrading damaged D1. We generated transformants of the cyanobacterium S ynechocystis sp. PCC6803 expressing GFP‐tagged versions of its four FtsH proteases. The ftsH2–gfp strain was functional for PSII repair under our conditions. Confocal microscopy shows that FtsH1 is mainly in the cytoplasmic membrane, while the remaining FtsH proteins are in patches either in the thylakoid or at the interface between the thylakoid and cytoplasmic membranes. HL exposure which increases the activity of the Photosystem II repair cycle led to no detectable changes in FtsH distribution, with the FtsH2 protease involved in D1 degradation retaining its patchy distribution in the thylakoid membrane. We discuss the possibility that the FtsH2–GFP patches represent Photosystem II ‘repair zones’ within the thylakoid membranes, and the possible advantages of such functionally specialised membrane zones. Anti‐GFP affinity pull‐downs provide the first indication of the composition of the putative repair zones.

Effects of cadmium metal on young gametophytes of Gelidium floridanum: metabolic and morphological changes.

By evaluating carotenoid content, photosynthetic pigments and changes in cellular morphology, growth rates, and photosynthetic performance, this study aimed to determine the effect of cadmium (Cd) on the development of young gametophytes of Gelidium floridanum. Plants were exposed to 7.5 and 15 μM of Cd for 7 days. Control plants showed increased formation of new filamentous thallus, increased growth rates, presence of starch grains in the cortical and subcortical cells, protein content distributed regularly throughout the cell periphery, and intense autofluorescence of chloroplasts. On the other hand, plants treated with Cd at concentrations of 7.5 and 15 μM showed few formations of new thallus with totally depigmented regions, resulting in decreased growth rates. Plants exposed to 7.5 μM Cd demonstrated alterations in the cell wall and an increase in starch grains in the cortical and subcortical cells, while plants exposed to 15 μM Cd showed changes in medullary cells with no organized distribution of protein content. The autofluorescence and structure of chloroplasts decreased, forming a thin layer on the periphery of cells. Cadmium also affected plant metabolism, as visualized by a decrease in photosynthetic pigments, in particular, phycoerythrin and phycocyanin contents, and an increase in carotenoids. This result agrees with decreased photosynthetic performance and chronic photoinhibition observed after treatment with Cd, as measured by the decrease in electron transport rate. Based on these results, it was concluded that exposure to Cd affects cell metabolism and results in significant toxicity to young gametophytes of G. floridanum.

ECOPHYSIOLOGICAL TRAITS OF ADULT TREES OF CRIOLLO COCOA CULTIVARS (THEOBROMA CACAOL.) FROM A GERMPLASM BANK IN VENEZUELA

SUMMARYWe studied physiological traits of 12 Criollo cocoa cultivars growing in a germplasm bank in the southern region of Maracaibo Lake Basin, during the rainy (RS) and dry seasons (DS) of 2007. A further evaluation of photosynthetic responses to changes in environmental parameters was done on three cultivars: Los Caños 001 (LCA001), Sur Porcelana 010 (SP010) and Escalante 001 (ESC001) in 2009 and 2010. Leaf water potential (ΨL) of most cultivars decreased during the DS of 2007, with the exception of ESC001. Maximum photosynthetic rate (Amax), stomatal conductance and water use efficiency varied among cultivars and seasons. The CO2-saturated photosynthetic rate (ACO2sat) was higher in LCA001 and ESC001 than in SP010, with no differences in carboxylation efficiency. Light curve responses of the three cultivars were similar. In all cultivars, no evidence of chronic photoinhibition was observed, since maximum quantum yield of photosystem II was high (0.77–0.81). We conclude that ESC001 has the best physiological performance (ΨLremained unchanged, highest Amax, ACO2satand photochemical activity), and it seems to be a promising cultivar for cocoa agroforestry systems in the southern region of Maracaibo.

Glycinebetaine Improved Photosynthesis in Canola under Salt Stress: Evaluation of Chlorophyll Fluorescence Parameters as Potential Indicators

AbstractSalinity is a widespread soil problem limiting productivity of oilseed crops worldwide. Reduction in growth generally associated with decline in photosynthesis. Accumulation of osmo‐protectants such as glycinebetaine (GB) permits the plant to cope with salt stress. This study was aimed to understand the mechanism by which exogenous GB application improves photosynthetic capacity and salt tolerance in two diverse lines of canola differing in salt tolerance using chlorophyll fluorescence technique. Glycinebetaine was applied as foliarly or through rooting medium to two canola lines Dunkeld and Cyclone that were grown under non‐saline or saline conditions. Root‐applied GB caused adverse effects whereas foliar application of GB was effective in improving growth of canola cultivars. However, the response of salt‐tolerant cultivar Dunkeld was stronger than that of salt sensitive Cyclone. Foliar application of GB improved accumulation of proline and plant water status. Application of GB enhanced the photosynthetic CO2 fixation, stomatal conductance (gs) and water‐use efficiency. The tolerant lines Dunkeld had more responsive to GB application. Chlorophyll fluorescence measurements (Fo, Fm, Fv, Fo/Fm, Fv/Fo and Fv/Fm ratios) revealed that salt stress reduced energy trapping efficiency by damaging oxygen evolving complex, over reduction of QA resulting in occurrence of chronic photoinhibition. However, exogenous GB protected the oxygen evolving centre of PSII and maintains activity of PSII. Although root‐applied GB adversely affected the growth of canola plants, it did not have any adverse effect on PSII photochemistry. Moreover, fluorescence parameters could provide a rapid means for determining salt tolerance in canola and could be a rapid and sensitive test to identify genotypes highly tolerant to salt stress.

Photosynthetic Light Utilization Efficiency, Water Relations and Leaf Growth of C3 and CAM Tropical Orchids under Natural Conditions

Native orchid species of Singapore in their natural conditions experience stress from high irradiance, high temperatures and periods of extended low rainfall, which impact orchid plant physiology and lead to reduced growth and productivity. In this study, it was found that there was a reduction in photochemical efficiency of photosystem II (PSII) in 6 native orchid species under high light (HL) and Bulbophyllum membranaceum under low light (LL). There was chronic photoinhibition in these 6 orchid species over a period of 3 months after transplanting onto the tree trunks without watering and fertilization, especially in Coelogynes mayeriana and Bulbophyllum membranaceum under both HL and LL. This chronic photoinhibition caused by sustained period of water deficit in their natural conditions was later reversed by natural re-watering conditions from higher rainfall. These results indicate that water deficit has a greater impact on photosynthetic light utilization efficiency than excess light. The present study also showed that after natural rewatering, relative water content (RWC) of leaves and pseudobulbs generally increased. During the natural re-watering, total leaf area also gradually increased and reached maximum expansion after 7 weeks under both HL and LL, with some exceptions due to leaf abscission or decline in total leaf area, possibly a strategy for water conservation.

Orchid Conservation in Singapore under Natural Conditions: Responses of Grammatophyllum Speciosum to Growth Irradiances

Grammatophyllum speciosum orchid plants were grown both on the ground and on trees under high light (HL) and low light (LL) conditions in Pasir Ris Park, Singapore. This paper studied photosynthetic energy utilization investigated by chlorophyll (Chl) fluorescence and photosynthetic pigments; plant water status measured by leaf relative water content (RWC) and stomatal conductance (gs). All plants had midday leaf RWC greater than 80%, indicating no severe water deficit occurred in any plants. Except for plants grown on trees under LL, midday depression of Fv/Fm ratios occurred in all other plants, indicating dynamic photoinhibiton. There was no chronic photoinhibition in any plants as Fv/Fm ratios recovered to healthy level at early morning. Grown on the ground under HL, G. speciosum had higher photochemical quantum yield (ΔF/Fm’), greater electron transport rate (ETR) and non-photochemical quenching (NPQ), indicating their higher efficiency in utilization and dissipation excess energy. Plants grown on the ground under HL had lower Chl and Chl/Car ratio, but higher content of carotenoids and Chl a/b ratio compared to the other plants, implying their acclimation to HL conditions. The highest gs was recorded at 1000 h, suggesting the avoidance of drought stress by widely opening stomata only for a short period of time in the early morning. Higher gs was related to higher leaf temperature, indicating that the role of gs in moderating leaf temperature. Understanding the effects of growth irradiance on photosynthesis of G. speciosum would be an important step towards a sustainable conservation of native orchids in Singapore.

Antioxidant and photoprotective defenses in response to gradual water stress under low and high irradiance in two Malvaceae tree species used for tropical forest restoration

This study provides a mechanistic basis of the tolerance to light and water stress in two tropical tree species of the Malvaceae used for the reforestation in the tropics. G. ulmifolia is more sensitive to high irradiance than C. speciosa . Thus, C. speciosa is more appropriate for the reforestation of degraded areas with potential excess light or water deficit. Reforestation programs in tropics are necessary to mitigate the impacts of climate change, the preservation of biodiversity and for recovery of degraded areas. However, little is known about the abiotic stress tolerance of the major tropical tree species used in reforestation. This study was aimed at evaluating antioxidant defenses and photoprotective pigments against excess light and water deficit in Guazuma ulmifolia Lam. and Ceiba speciosa (A.St.-Hil) Ravenna. Plantlets of 6-month-old were exposed to progressive water deficit by withholding water under high and low irradiance growth conditions and then re-irrigated. G. ulmifolia leaves under high irradiance showed increased photo-oxidative damage, as indicated by a decreased Fv/Fm ratio and 5−12-fold increases in MDA levels, which correlated with a 14-fold increase in SOD activity, a 90 % decrease in POD activity and a 3−4-fold increases in hydrogen peroxide levels. Water deficit combined with high irradiance caused stronger chronic photoinhibition, indicating a synergistic effect that cannot be counterbalanced by antioxidant or photo-protective mechanisms. In contrast, to avoid photo-oxidative damage, C. speciosa enhanced SOD activity, maintained POD and CAT activities, and increased xanthophyll cycle pool size and DPS and β-carotene accumulation. These results indicate that G. ulmifolia is more sensitive to high irradiance than C. speciosa at early stages of development. Thus, C. speciosa is more appropriate for the reforestation of degraded areas with potential excess light or water deficit. In conclusion, this study provides a mechanistic basis of the tolerance abiotic stress in two tropical trees, thereby contributing for management strategies in reforestation projects.

Can increased glacial melting resulting from global change provide attached algae with transient protection against high irradiance?

Summary Climate change is altering temperatures and precipitation patterns all over the world. Melting glaciers increase surface run‐off, thereby increasing the transport of suspended solids through streams. The increased load of suspended solids affects turbidity, which decreases the availability of photosynthetically active radiation for primary producers. We analysed how glacial loading of clays influences the light : nutrient ratio and photosynthetic parameters (measured with a pulse amplitude modulated fluorometer) and the carbon : phosphorus (C : P) elemental ratio of periphytic primary producers. A field study was conducted in two canopy‐free streams that receive water from the glaciers of Mount Tronador (Patagonia, Argentina), one with high glacial load and the other with clear water. In addition, we conducted an in situ colonisation experiment with three different light treatments. We observed that periphytic biomass (chlorophyll a and carbon content) in the streams and in the experiment varied directly with turbidity. Moreover, photosynthetic parameters varied similarly because of an increase in the efficiency of electron transfer per open reactive centre in the more turbid stream and a chronic photoinhibition of photosystem II in the clearer stream. Periphytic C : P also varied with turbidity as we observed a decrease in C : P with an increase in light in both streams and in the experiment. Our main conclusion is that an increase in glacial melting with its associated increase in glacial clay load should protect primary producers against high irradiances (photosynthetic active radiation + ultraviolet radiation) in canopy‐free streams.

Investigating the photoprotective role of cytochrome b-559 in photosystem II in a mutant with altered ligation of the haem.

Despite many years of study, the physiological role of cytochrome b-559 (Cyt b-559) within the photosystem II (PSII) complex still remains unclear. Here we describe the analysis of a mutant of the green alga Chlamydomonas reinhardtii in which the His ligand to the haem, provided by the alpha subunit, has been replaced by a Cys residue. The mutant is unable to grow photoautotrophically but can assemble oxygen-evolving PSII supercomplexes to 15-20% of the levels found in the wild-type control. Haem is still detected in the isolated PSII supercomplexes but at sub-stoichiometric levels consistent with weaker binding to the mutated cytochrome. Analysis of PSII activity in cells indicates slowed electron transfer in the mutant between plastoquinones QA and QB. We show that PSII activity in the mutant is more sensitive to chronic photoinhibition than the WT control because of two effects: a faster rate of damage and an impaired PSII repair cycle at the level of synthesis and/or incorporation of D1 into PSII. We also demonstrate that Cyt b-559 plays a role during the critical stage of assembling the Mn4CaO5 cluster. Overall we conclude that Cyt b-559 optimises electron transfer on the acceptor side of PSII and plays physiologically important roles in the assembly, repair and maintenance of the complex.

Rootstock alleviates PEG-induced water stress in grafted pepper seedlings: Physiological responses

Recent studies have shown that tolerance to abiotic stress, including water stress, is improved by grafting. In a previous work, we took advantage of the natural variability of Capsicum spp. and selected accessions tolerant and sensitive to water stress as rootstocks. The behavior of commercial cultivar 'Verset' seedlings grafted onto the selected rootstocks at two levels of water stress provoked by adding 3.5 and 7% PEG (polyethylene glycol) was examined over 14 days. The objective was to identify the physiological traits responsible for the tolerance provided by the rootstock in order to determine if the tolerance is based on the maintenance of the water relations under water stress or through the activation of protective mechanisms. To achieve this goal, various physiological parameters were measured, including: water relations; proline accumulation; gas exchange; chlorophyll fluorescence; nitrate reductase activity; and antioxidant capacity. Our results indicate that the effect of water stress on the measured parameters depends on the duration and intensity of the stress level, as well as the rootstock used. Under control conditions (0% PEG) all plant combinations showed similar values for all measured parameters. In general terms, PEG provoked a strong decrease in the gas exchange parameters in the cultivar grafted onto the sensitive accessions, as also observed in the ungrafted plants. This effect was related to lower relative water content in the plants, provoked by an inefficient osmotic adjustment that was dependent on reduced proline accumulation. At the end of the experiment, chronic photoinhibition was observed in these plants. However, the plants grafted onto the tolerant rootstocks, despite the reduction in photosynthetic rate, maintained the protective capacity of the photosynthetic machinery mediated by osmotic adjustment (based on higher proline content). In addition, water stress limited uptake and further NO3(-) transfer to the leaves. Increased nitrate reductase activity in the roots was observed, mainly in plants grafted onto the sensitive rootstocks, as well as the ungrafted plants, and this was associated with the lessened flux to the leaves. This study suggests that PEG-induced water stress can be partially alleviated by using tolerant accessions as rootstocks.

Photoacclimation characteristics of Sargassum thunbergii germlings under different light intensities

The photosynthesis and growth responses of Sargassum thunbergii germlings to different light intensities (10, 60, and 300 μmol photons m−2 s−1) were investigated. Maximum photochemical efficiency (Fv/Fm), rapid light curves (RLCs), and photochemical and non-photochemical quenching (qP and NPQ) were estimated by a pulse amplitude-modulated fluorometer. The photosynthesis of S. thunbergii germlings exhibited different properties to optimize light capture and utilization. The excitation pressure (1 − qP) was rapidly increased to approximately 0.27 showing that germlings responded to high light by chronic photoinhibition with an accumulation of closed reaction centers, which ultimately resulted in a slow growth. This was accompanied by a reduced Fv/Fm with time and a development of high capacity for NPQ. Although Fv/Fm in moderate-light germlings did not fully recover overnight, germlings demonstrated a less severe chronic photoinhibition considering the reduced degree of excitation pressure accumulation of approximately 0.15. The relative stability of photosynthetic capacity (rETRmax, Ek, and α) could endow germlings with the highest relative growth rate (RGR) of approximately 9.3 % day−1 in moderate light. By contrast, low-light germlings demonstrated high Fv/Fm and Fo, corresponding high α collectively suggested greater efficiency of light absorption and energy transformation. Sustained increases in electron transport capacity (rETRmax and Ek) occurred in low-light germlings, which resulted in a stable RGR of over 8.2 % day−1. Consequently, S. thunbergii germlings are considered to prefer low light regimes and have a relative capacity of moderate and high light tolerance. However, the light acclimation to oversaturating conditions is at the cost of slow growth to maintain survival.

Gender, mediterranean drought, and seasonality: photosystem II photochemistry in Pistacia lentiscus L.

In this work, photosystem II (PSII) photochemistry, leaf water potential, and pigment contents of male and female Pistacia lentiscus L. were investigated during a seasonal cycle at three different, arid locations: superior semiarid, inferior semiarid, and arid. The results showed that the gender, season, and the site conditions interacted to influence the quantum yield and pigment contents in P. lentiscus. Predawn leaf water status was determined only by the site and season. The annual patterns of PSII maximum quantum efficiency (Fv/Fm) were characterized by a suboptimal activity during the winter, especially, populations with the more negative water potential exhibited a lower chlorophyll (Chl) a content and chronic photoinhibition irrespective of a gender. We also demonstrated that both photochemical or nonphotochemical mechanisms were involved to avoid the photoinhibition and both of them depended on the season. This plasticity of photosynthetic machinery was accompanied by changes in carotenoids and Chl balance. In the spring, the female Fv/Fm ratio was significantly higher than in male individuals, when the sexual dimorphism occurred during the fruiting stage, regardless of site conditions. P. lentiscus sex-ratio in Mediterranean areas, where precipitations exceeded 500 mm, was potentially female-biased. Among the fluorescence parameters investigated, nonphotochemical quenching coefficient appeared as the most useful one and a correlation was found between Chl a content and Fv/Fm. These results suggest that functional ecology studies would be possible on a large scale through light reflectance analysis.

The soil hydrogel improved photosynthetic performance of beech seedlings treated under drought

The effect of soil amendment with the STOCKOSORB®500 MICRO hydrophilic polymer on the photosynthetic traits in beech seedlings (Fagus sylvatica L.) during 50 days of dehydration was investigated. Dehydration was detected through osmotic potential (Ψ<sub>s</sub>) in the assimilatory organs of beech seedlings. The addition of Stockosorb positively affected the CO<sub>2</sub> assimilation rate (A) and instantaneous water use efficiency (A/T), for severely drought-treated seedlings. In comparison with irrigated plants, the values of A of non-irrigated plants with Stockosorb substrate decreased by 50%, and in non-irrigated plants with common substrate by 88%. The fast kinetics of chlorophyll a fluorescence indicated chronic photoinhibition under drought treatment without Stockosorb, while no significant changes in maximal quantum efficiency (F<sub>v</sub>/F<sub>m</sub>) were recorded under drought treatment with Stockosorb. The actual quantum efficiency of PSII (Φ<sub>PSII</sub>) markedly decreased in both treatments – with and without Stockosorb, though significant differences were found only between control treatments and drought treatment without Stockosorb. Moreover, the thermal energy dissipation (NPQ) was strongly limited under severe drought stress. The capacity to down regulate PSII functionality through non-photochemical quenching was maintained under drought treatment with Stockosorb. The results indicate that an amendment with soil conditioner significantly improved the photosynthetic performance of drought-stressed beech seedlings.

Root-zone CO<sub>2</sub> and root-zone temperature effects on photosynthesis and nitrogen metabolism of aeroponically grown lettuce (Lactuca sativa L.) in the tropics

Effects of elevated root-zone (RZ) CO2 concentration (RZ [CO2]) and RZ temperature (RZT) on photosynthesis, productivity, nitrate (NO3 −), total reduced nitrogen (TRN), total leaf soluble and Rubisco proteins were studied in aeroponically grown lettuce plants in a tropical greenhouse. Three weeks after transplanting, four different RZ [CO2] concentrations (ambient, 360 ppm, and elevated concentrations of 2,000; 10,000; and 50,000 ppm) were imposed on plants at 20°C-RZT or ambient(A)-RZT (24–38°C). Elevated RZ [CO2] resulted in significantly higher light-saturated net photosynthetic rate, but lower light-saturated stomatal conductance. Higher elevated RZ [CO2] also protected plants from both chronic and dynamic photoinhibition (measured by chlorophyll fluorescence Fv/Fm ratio) and reduced leaf water loss. Under each RZ [CO2], all these variables were significantly higher in 20°C-RZT plants than in A-RZT plants. All plants accumulated more biomass at elevated RZ [CO2] than at ambient RZ [CO2]. Greater increases of biomass in roots than in shoots were manifested by lower shoot/root ratios at elevated RZ [CO2]. Although the total biomass was higher at 20°C-RZT, the increase in biomass under elevated RZ [CO2] was greater at A-RZT. Shoot NO3 − and TRN concentrations, total leaf soluble and Rubisco protein concentrations were higher in all elevated RZ [CO2] plants than in plants under ambient RZ [CO2] at both RZTs. Under each RZ [CO2], total leaf soluble and Rubisco protein concentrations were significantly higher at 20°C-RZT than at A-RZT. Our results demonstrated that increased P Nmax and productivity under elevated [CO2] was partially due to the alleviation of midday water loss, both dynamic and chronic photoinhibition as well as higher turnover of Calvin cycle with higher Rubisco proteins.

Prolonging the hydration and active metabolism from light periods into nights substantially enhances lichen growth

This study investigates how hydration during light and dark periods influences growth in two epiphytic old forest lichens, the green algal Lobariapulmonaria and the cyanobacterial L.scrobiculata. The lichens were cultivated in growth chambers for 14days (200μmolm(-1)s(-2); 12h photoperiod) at four temperature regimes (25/20°C, 21/16°C, 13/8°C, and 6/1°C; day/night temperatures) and two hydration regimes (12hday-time hydration; 12hday-time+12h night-time hydration). Growth was highly dynamic, showing that short-term growth experiments in growth cabinets have a high, but largely unexplored potential in functional lichen studies. The highest measured growth rates were not far from the maximal dry matter gain estimated from published net photosynthetic CO2 uptake data. For the entire data set, photobiont type, temperature, hydration regime and specific thallus mass accounted for 46.6% of the variation in relative growth rate (RGR). Both species showed substantially higher relative growth rates based on both biomass (RGR) and thallus area (RTAGR) when they were hydrated day and night compared to hydration in light only. Chronic photoinhibition was substantial in thalli hydrated only during the day time and kept at the highest and lowest temperature regimes, resulting in exponential increases in RGR with increasing maximal PSII efficiency (F v/F m) in both species. However, the depression in F v/F m was stronger for the cyanolichen than for the cephalolichen at extreme temperatures. The growth-stimulating effect of night-time hydration suggests that nocturnal metabolic activity improves recovery of photoinhibition and/or enhances the conversion rate of photosynthates into thallus extension.

Field analysis of photoprotection in co‐occurring cool climateC3andC4grasses

C(4) photosynthesis is particularly successful at high light intensities and high temperatures, but is relatively rare when the average growing season temperature is less than about 15°C. We tested the hypothesis that rapidly reversible photoprotection enables some C(4) species to tolerate cool climates, by focusing on two questions: (1) Do chlorophyll fluorescence responses differ seasonally between co-occurring C(3) and C(4) grasses in the field? (2) Does xanthophyll-mediated photoprotection differ between the two pathways? Spartina pectinata (C(4) ) and Calamogrostis canadensis (C(3) ) were sampled in a herbaceous fresh-water meadow in New Brunswick, Canada (45°N 66°W). Non-photochemical thermal energy dissipation (Φ(NPQ) ) and the epoxidation state of the xanthophyll cycle (EPS) were used as indicators of photoprotection. We observed no differential susceptibility to chronic photoinhibition (i.e. photodamage) between the C(3) and C(4) species, except potentially during spring emergence. On average, C. canadensis showed higher levels of protective dynamic photoinhibition throughout the growing season, but S. pectinata had greater Φ(NPQ) and lower EPS during seasonal and daily temperature minima. The low Rubisco capacity of C(4) species is a potential limiting factor to C(4) success at high latitudes, but our findings suggest that it is unlikely via a photoinhibitory feedback mechanism.

Seasonal variations of photosynthesis, gas exchange, quantum efficiency of photosystem II and biochemical responses of Jatropha curcas L. grown in semi-humid and semi-arid areas subject to water stress

This study aimed to compare the physiological adaptations of Jatropha curcas in two regions with different climates, the Agreste (a semi-arid tropical climate) and the Florest zone (a semi-humid tropical climate) subject to water stress. Measurements of gas exchange and photochemical and biochemical efficiency were performed in two seasons, the dry season (September to February) and the rainy season (April and August), between 2006 and 2007. During the dry season in the Florest zone, the J. curcas plants presented better photosynthetic performance, with smaller decreases in the rates of photosynthesis, stomatal conductance and transpiration because the conditions of water availability in the soil were slightly higher than in the semi-arid region. A greater vapor pressure deficit was observed during the dry season of the semi-arid region, showing a negative correlation with the stomatal opening of the leaves, which contributed to a reduction in the values of the net photosynthetic rate. During the dry season in the semi-arid region, dynamic photoinhibition occurred in the leaves of J. curcas around mid-day, with a rapid recovery during the afternoon. However, chronic photoinhibition may have occurred in some plants with a severe water deficit. A reduction in the effective quantum efficiency was also observed during the driest months. The increase of catalase activity and the accumulation of proline, total soluble sugars and amino acids during periods of low water availability in the soil suggest that J. curcas presents an efficient protection mechanism that allows the plant to survive under drought conditions.

Frost-Acclimation of Photosynthesis in Overwintering Mediterranean Holm Oak, Grown in Central Europe

As a consequence of global change, forestry in Central Europe has to expect and be prepared for an increase of hot and dry summers in the near future. In two model plantations of the Mediterranean holm oak (Quercus ilex L.) in Central Europe (Rhine-Main basin) we tested its potential as a future forestry tree for drought-threatened stands by studying its overwintering strategy under harsh winter conditions. During prolonged frost periods, chronic photoinhibition was developed, which lasted until the end of the frost period. Nearly all plants survived minimum temperatures of −16 to −18 °C and their photosynthetic apparatus recovered completely during late winter. A detailed study of the temperature dependence of chlorophyll (chl) fluorescence parameters of the OJIP test revealed statistically significant correlations between minimum temperature and maximum quantum yield of primary photochemistry (Fv/Fm), absorption rate/reaction centre (ABS/RC), dissipation rate/reaction centre (DI0/RC) and electron transport rate/reaction centre (ET0/RC) as well as with the deepoxidation state (DES) of the xanthophyll pigments. The DES correlated with Fv/Fm, ABS/RC, DI0/RC and ET0/RC. It is concluded, that from the point of view of the winter hardiness of the photosynthetic apparatus, Q. ilex should be further investigated as a potential future forestry tree also for very dry and warm stands in Central Europe under the scenarios of climate change.

Patterns of spatio‐temporal distribution of winter chronic photoinhibition in leaves of three evergreen Mediterranean species with contrasting acclimation responses

High irradiance and relatively low temperature, which characterize Mediterranean winters, cause chilling stress in plants. Downregulation of photosynthetic efficiency is a mechanism that allows plants to survive these conditions. This study aims to address whether this process shows a regular spatial pattern across leaf surface or not. Three species (Buxus sempervirens, Cistus albidus and Arctostaphylos uva-ursi) with contrasting responses to winter stress were studied. During 7 days, macro and micro Fv/Fm spatial patterns were monitored by the use of chlorophyll fluorescence imaging techniques. In the field, the strongest photoinhibition was found in B. sempervirens, while there was almost no chronic photoinhibition in C. albidus. In leaves of the first species, Fv/Fm decreased from base to tip while in C. albidus it was uniform over the leaf lamina. An intermediate behavior is shown by A. uva-ursi leaves. Spatial heterogeneity distribution of Fv/Fm was found inside the leaves, resulting in greater Fv/Fm values in the inner layers than in the outer ones. Neither xanthophyll-linked downregulation of Fv/Fm nor protein remobilization were the reasons for such spatial patterns since pigment composition and nitrogen content did not reveal tip-base differences. During recovery from winter, photoinhibition changes occurred in Fv/Fm, pigments and chloroplast ultrastructure. This work shows for the first time that irrespective of physiological mechanisms responsible for development of winter photoinhibition, there is an acclimation response with strong spatio-temporal variability at leaf level in some species. This observation should be taken into account when modeling or scaling up photosynthetic responses.